1 //===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass eliminates machine instruction PHI nodes by inserting copy
11 // instructions. This destroys SSA information, but is the desired input for
12 // some register allocators.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/CodeGen/LiveVariables.h"
17 #include "llvm/CodeGen/Passes.h"
18 #include "llvm/CodeGen/MachineFunctionPass.h"
19 #include "llvm/CodeGen/MachineInstr.h"
20 #include "llvm/CodeGen/SSARegMap.h"
21 #include "llvm/Target/TargetInstrInfo.h"
22 #include "llvm/Target/TargetMachine.h"
23 #include "llvm/ADT/DenseMap.h"
24 #include "llvm/ADT/STLExtras.h"
28 struct PNE : public MachineFunctionPass {
29 bool runOnMachineFunction(MachineFunction &Fn) {
32 // Eliminate PHI instructions by inserting copies into predecessor blocks.
33 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
34 Changed |= EliminatePHINodes(Fn, *I);
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 AU.addPreserved<LiveVariables>();
41 MachineFunctionPass::getAnalysisUsage(AU);
45 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
46 /// in predecessor basic blocks.
48 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
51 RegisterPass<PNE> X("phi-node-elimination",
52 "Eliminate PHI nodes for register allocation");
56 const PassInfo *llvm::PHIEliminationID = X.getPassInfo();
58 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
59 /// predecessor basic blocks.
61 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
62 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
63 return false; // Quick exit for normal case...
65 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
66 const TargetInstrInfo &MII = *MF.getTarget().getInstrInfo();
67 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
69 // VRegPHIUseCount - Keep track of the number of times each virtual register
70 // is used by PHI nodes in successors of this block.
71 DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount;
72 VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg());
74 unsigned BBIsSuccOfPreds = 0; // Number of times MBB is a succ of preds
75 for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(),
76 E = MBB.pred_end(); PI != E; ++PI)
77 for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(),
78 E = (*PI)->succ_end(); SI != E; ++SI) {
79 BBIsSuccOfPreds += *SI == &MBB;
80 for (MachineBasicBlock::iterator BBI = (*SI)->begin(); BBI !=(*SI)->end() &&
81 BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
82 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
83 VRegPHIUseCount[BBI->getOperand(i).getReg()]++;
86 // Get an iterator to the first instruction after the last PHI node (this may
87 // also be the end of the basic block). While we are scanning the PHIs,
88 // populate the VRegPHIUseCount map.
89 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
90 while (AfterPHIsIt != MBB.end() &&
91 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
92 ++AfterPHIsIt; // Skip over all of the PHI nodes...
94 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
95 // Unlink the PHI node from the basic block, but don't delete the PHI yet.
96 MachineInstr *MPhi = MBB.remove(MBB.begin());
98 assert(MRegisterInfo::isVirtualRegister(MPhi->getOperand(0).getReg()) &&
99 "PHI node doesn't write virt reg?");
101 unsigned DestReg = MPhi->getOperand(0).getReg();
103 // Create a new register for the incoming PHI arguments
104 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
105 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
107 // Insert a register to register copy in the top of the current block (but
108 // after any remaining phi nodes) which copies the new incoming register
109 // into the phi node destination.
111 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
113 // Update live variable information if there is any...
115 MachineInstr *PHICopy = prior(AfterPHIsIt);
117 // Add information to LiveVariables to know that the incoming value is
118 // killed. Note that because the value is defined in several places (once
119 // each for each incoming block), the "def" block and instruction fields
120 // for the VarInfo is not filled in.
122 LV->addVirtualRegisterKilled(IncomingReg, PHICopy);
124 // Since we are going to be deleting the PHI node, if it is the last use
125 // of any registers, or if the value itself is dead, we need to move this
126 // information over to the new copy we just inserted.
128 LV->removeVirtualRegistersKilled(MPhi);
130 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
131 RKs = LV->dead_range(MPhi);
132 if (RKs.first != RKs.second) {
133 for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
134 LV->addVirtualRegisterDead(*I, PHICopy);
135 LV->removeVirtualRegistersDead(MPhi);
139 // Adjust the VRegPHIUseCount map to account for the removal of this PHI
141 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
142 VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= BBIsSuccOfPreds;
144 // Now loop over all of the incoming arguments, changing them to copy into
145 // the IncomingReg register in the corresponding predecessor basic block.
147 for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) {
148 MachineOperand &opVal = MPhi->getOperand(i-1);
150 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
151 // source path the PHI.
152 MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMachineBasicBlock();
154 MachineBasicBlock::iterator I = opBlock.getFirstTerminator();
156 // Check to make sure we haven't already emitted the copy for this block.
157 // This can happen because PHI nodes may have multiple entries for the
158 // same basic block. It doesn't matter which entry we use though, because
159 // all incoming values are guaranteed to be the same for a particular bb.
161 // If we emitted a copy for this basic block already, it will be right
162 // where we want to insert one now. Just check for a definition of the
163 // register we are interested in!
165 bool HaveNotEmitted = true;
167 if (I != opBlock.begin()) {
168 MachineBasicBlock::iterator PrevInst = prior(I);
169 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
170 MachineOperand &MO = PrevInst->getOperand(i);
171 if (MO.isRegister() && MO.getReg() == IncomingReg)
173 HaveNotEmitted = false;
179 if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
180 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) &&
181 "Machine PHI Operands must all be virtual registers!");
182 unsigned SrcReg = opVal.getReg();
183 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
185 // Now update live variable information if we have it.
187 // We want to be able to insert a kill of the register if this PHI
188 // (aka, the copy we just inserted) is the last use of the source
189 // value. Live variable analysis conservatively handles this by
190 // saying that the value is live until the end of the block the PHI
191 // entry lives in. If the value really is dead at the PHI copy, there
192 // will be no successor blocks which have the value live-in.
194 // Check to see if the copy is the last use, and if so, update the
195 // live variables information so that it knows the copy source
196 // instruction kills the incoming value.
198 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
200 // Loop over all of the successors of the basic block, checking to see
201 // if the value is either live in the block, or if it is killed in the
202 // block. Also check to see if this register is in use by another PHI
203 // node which has not yet been eliminated. If so, it will be killed
204 // at an appropriate point later.
206 bool ValueIsLive = false;
207 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
208 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
209 MachineBasicBlock *SuccMBB = *SI;
211 // Is it alive in this successor?
212 unsigned SuccIdx = SuccMBB->getNumber();
213 if (SuccIdx < InRegVI.AliveBlocks.size() &&
214 InRegVI.AliveBlocks[SuccIdx]) {
219 // Is it killed in this successor?
220 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
221 if (InRegVI.Kills[i]->getParent() == SuccMBB) {
226 // Is it used by any PHI instructions in this block?
228 ValueIsLive = VRegPHIUseCount[SrcReg] != 0;
231 // Okay, if we now know that the value is not live out of the block,
232 // we can add a kill marker to the copy we inserted saying that it
233 // kills the incoming value!
236 MachineBasicBlock::iterator Prev = prior(I);
237 LV->addVirtualRegisterKilled(SrcReg, Prev);
239 // This vreg no longer lives all of the way through opBlock.
240 unsigned opBlockNum = opBlock.getNumber();
241 if (opBlockNum < InRegVI.AliveBlocks.size())
242 InRegVI.AliveBlocks[opBlockNum] = false;
248 // Really delete the PHI instruction now!